1. Field of the Invention
This invention pertains to the manufacture of printed circuits, particularly to a fixture for holding flexible printed circuits during the time that such boards are being processed, for example, having molten solder removed. In such manufacture, a plastic substrate contains conductive areas and plated throughgoing holes, in order to receive leads from electrical components which are to be placed on and soldered to the printed circuit.
2. Description of the Prior Art
U.S. Pat. No. 3,865,298 discloses an apparatus used in the manufacture of rigid printed circuits. In this apparatus, a plastic substrate containing conductive areas and plated throughgoing holes is fluxed with a suitable flux by, e.g., dipping, swabbing, or immersion. Subsequently, the fluxed plastic substrate is contacted with molten solder by, e.g., immersion into a molten solder bath.
After this latter immersion in the molten solder bath, the semi-finished printed circuit is withdrawn from the bath and is subjected to streams of hot gas, preferably hot air. These streams of hot gas perform the functions of clearing excess solder from the holes in the substrate and leveling the layers of solder on the semi-finished printed circuit so that a solder layer of a uniform thickness is produced.
Within the apparatus disclosed in the above-mentioned U.S. Pat. No. 3,865,298, the semi-finished printed circuit travels in a well-defined plane. The streams of hot gas are produced by hot air knives which are precisely situated relative to the path traversed by the semi-finished printed circuit. In the event that the semi-finished printed circuit did not move along a well-defined path, the distances between the semi-finished circuit and the hot air knives would vary excessively. This variation would make the solder thickness less uniform, and might leave at least some of the holes obstructed by excess solder.
The apparatus disclosed in this reference is adequate for removing solder from conventional rigid substrates. However, when used to remove solder from flexible substrates, this apparatus produces unsatisfactory results. When flexible substrates are subjected to the streams of hot gas, they do not remain planar. Therefore, they do not travel along a precise and well-defined path, so that the layer of solder is irregular in thickness and the holes in the flexible substrate are sometimes blocked by solder. These problems cannot be solved by supporting the edges of the flexible substrate alone, because flexing will take place intermediate any points or lines of such edge support.
Therefore, it would be advantageous to provide an apparatus and a process which would permit solder removal from flexible substrates using the techniques disclosed in the above-mentioned reference.